The present invention generally relates to optical communication systems and more particularly to an apparatus for selecting narrowly spaced optical channels in a dense wavelength division multiplexed transmission system.
Wavelength division multiplexing (WDM) is a technique for increasing the capacity of existing fiber optic networks by transmitting a plurality of channels over a single waveguide medium. Dense WDM (DWDM) systems are also being employed where the channel spacings are more narrow than WDM systems, thereby providing increased signal traffic over the same waveguide. In these types of systems, it""s essential to provide an optical device which selects a particular channel having a corresponding wavelength from a plurality of closely spaced channels associated with a multiplexed optical signal.
Channel selectivity can be performed by various techniques one of which is through the use of fiber Bragg gratings. A Bragg grating comprises a series of photoinduced refractive index perturbations in an optical fiber which reflects optical signals within a selected wavelength band and transmits wavelengths outside of the selected wavelength band. Essentially, a Bragg grating is a reflection filter because of the presence of what is known as a xe2x80x9cstop bandxe2x80x9d which is the region where most of the incident light is reflected. The stop band is generally centered at the Bragg wavelength defined as xcexBRAGG=2xc3x1xcex9, where xc3x1 is the modal index and xcex9 is the grating period. Bragg gratings are described in more detail in Morey et al., Photoinduced Bragg Gratings in Optical Fibers, Optics and Photonics News, February 1994, pp. 9-14, and A. M. Vengsarkar et al., Long-Period Fiber Gratings As Band-Rejection Filters, Journal of Lightwave Technology, vol. 14, no. 1, January 1996, pp. 58-65, the disclosures of which are incorporated herein by reference.
For a typical Bragg grating, the refractive index varies over the fiber length. The periodic variation in refractive index can take the form of a series of xe2x80x9cpeaksxe2x80x9d and xe2x80x9cvalleysxe2x80x9d, whereby the distance or period between two adjacent refractive index peaks defines, in part, the wavelength to be reflected by the Bragg grating. The bandwidth of a fiber Bragg grating is inversely proportional to its length. That is, the longer the grating, the more narrow the bandwidth to be reflected.
There are various methods for writing fiber Bragg gratings, the most common of which uses two interfering UV beams focused onto a fiber core. The light intensity modulation generated by the interfering UV beams alters the refractive index of the core material thereby generating a refractive index modulation pattern in the fiber. The resulting Bragg grating has a transmission minimum or stop-band at or near a desired wavelength where the incident light is reflected by the grating. When writing a Bragg grating, the refractive index distribution is responsible for the magnitude of sidebands in the spectral response. A truly apodized grating has Gaussian refractive index depth modulation with constant average refraction over its entire length. Such a grating virtually eliminates any sidebands. This is desirable when writing gratings for very narrowly spaced channels, for example in DWDM communication systems where crosstalk from closely spaced adjacent channels is at issue.
Although fiber Bragg gratings can be readily modeled using coupled mode equation theory, producing these gratings becomes more difficult with decreasing bandwidth, i.e. when selecting optical channels within a narrowly spaced channel plan. Relatively long gratings are difficult to produce mainly due to the stringent requirements for the homogeneity of the large diameter UV beams used to write the gratings and because of the sensitivity of the reflection spectrum caused by small aberrational effects from the associated optical components. In other words, the more narrow the bandwidth, the longer the grating and the more difficult it is to produce, especially in a mass production context.
Because of the need for wavelength selective components in WDM and DWDM systems with decreasing channel spacings, a narrow bandwidth filter device is needed to select particular optical channels within a closely spaced channel plan and to provide such a device which can be reliably reproduced for manufacturing purposes.
The present invention meets these needs and avoids the above-referenced drawbacks by providing an optical device which utilizes a first and second fiber Bragg grating which are shorter in length than a single longer grating otherwise used to select a narrowly spaced optical channel. The optical device in accordance with the present invention includes an optical transfer element having a first port for receiving a multiplexed optical signals having a plurality of channels, each of the channels at a respective wavelength. A first filtering element optically communicates with a second port of the transfer element. The first filtering element is configured to have a low transmissivity characteristic at or near at least one particular wavelength associated with a channel included in the plurality of multiplexed signals. The first filtering element receives the multiplexed signals and reflects a first portion of the multiplexed signals including the at least one channel toward the second port of the transfer element. A second filtering clement optically communicates with the third port of the transfer element. The second filtering element is configured to have a low transmissivity characteristic at or near the at least one channel. The second filtering element receives the first portion of the multiplexed signal and reflects the at least one optical channel toward the third port of the transfer element and transmits a second portion of the multiplexed signal which includes optical channels near the at least one optical channel. The transfer element supplies the at least one optical channel to said fourth port.
The foregoing, and other features and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims.